My gut says Nancy has done all this before, is probably doing it now with somebody else, and will continue to do this.
It's risky for sure, but if letting her husband know stops this craziness from happening to another man or woman, it'd be worth it to me.
Plus, as a husband, I think he has a right to know. If it was just an online affair, it would be one thing. This was just lie after lie to make Meg feel sorry for her, make her go through a gauntlet of emotions, and bring her to the very edge of leaving her family, only to recoil every time asked her to meet her.
If Nancy is already getting help, it won't be that much of a revelation to him. If she's completely lying to her husband too, it may stop her from doing this sort of thing any more.